1. Field of the Invention
The present invention relates to a one-dimensional or two-dimensional solid-state image pickup device, adapted for use in a facsimile apparatus, a digital copying machine or an X-ray image taking apparatus.
2. Related Background Art
In the image reading system of the facsimile apparatus, the digital copying machine or the X-ray image taking apparatus, there has generally been employed a system consisting of a combination of a reduction optical system and a CCD sensor. Also in recent years, owing to the development of photoelectric converting semiconductor materials represented by hydrogenated amorphous silicon (hereinafter represented as a-Si), there has been proposed a contact sensor containing the photoelectric converting elements or such elements and a signal processing unit on a large-sized substrate and adapted to read the information from a source with a real-size optical system, and such contact sensor is being commercialized particularly in the one-dimensional sensor. In particular, since a-Si can be utilized not only as the photoelectric converting material but also as the semiconductor material for a thin film field effect transistor (hereinafter represented as TFT), it allows to simultaneously form a photoelectric converting semiconductor layer and a semiconductor layer of the TFT on a same substrate, thereby simplifying the structure.
However the increase in the size of the substrate may result in a lowered production yield with respect to the number of processed substrates, because the satisfactory or defective product can only be judged after the final stage of the manufacturing process. Also there is required a large investment in the manufacturing equipment since exclusive process and manufacturing line, including the film formation and the photoresist process, have to be newly established corresponding to the substrate size. Based on such viewpoint, the realization of economically acceptable product poses a difficult aspect. In particular, an improvement in the manufacturing yield has to be considered sufficiently in case of the solid-state image pickup device of a large image reading area, since such device cannot be obtained, unlike the line sensors, in a large number from a single substrate.
For this reason, attention is being attracted, for the purpose of improving the manufacturing yield, to a method of obtaining a large-sized device by arranging a plurality of small-sized substrates. FIGS. 1A and 1B show an example of the solid-state image pickup device forming by arranging a plurality (2.times.2=4 units in the illustrated example) of small-sized substrates. In these drawings there are shown circuit boards 2001, flexible circuit boards 2002, lead electrodes 2010 formed on the sensor substrates, and sealant 2011.
The sealant 2011 is applied at the junction of the lead electrodes 2010 and the flexible circuit boards 2002, for the purpose of preventing corrosion of the lead electrodes 2010, and is generally composed of silicone resin. There are also shown sensor substrates 2003-2006 bearing thereon sensor elements 2007 (represented by sensor areas), a base member 2008 for supporting the sensor substrates 2003-2006, and an adhesive material 2009 for fixing the sensor substrates 2003-2006 to the base member 2008. The sensor substrates 2003-2006 are fixed to the base member 2008, with such alignment that the pitch of the pixels is matched two-dimensionally.
FIG. 2 shows an example of the process for preparing the solid-state image pickup device formed by arranging a plurality of small-sized substrates as shown in FIGS. 1A and 1B. In a step (2101), four sensor substrates 2110-2113 prepared by a thin-film semiconductor process are sliced into a desired size by a rotary diamond blade 2114.
Then, in a step (2102), the circuit board 2001 is connected, by suitable jointing means 2115 and through the flexible circuit board 2002, to the lead electrode portion 2010 of the sensor substrate 2110-2113 prepared in the predetermined size. In a next step (2103), the sealant 2011 is coated by a dispenser 2116 so as to cover the junction portion of the lead electrode portion 2010 and the flexible circuit board 2002.
Then, in a step (2104), the sensor substrates after sealing are mutually so aligned that the pitch of the pixels matches two-dimensionally, and the substrates are placed on and fixed to the base member 2008 coated with the adhesive material 2009 in advance. (In FIG. 2, the sensor elements 2007 are omitted from the illustration.)
In case the above-mentioned solid-state image pickup device is used as an X-ray image pickup device, there is adopted a configuration in which the X-ray is converted by a fluorescent plate into a sensitive wavelength range of the sensor, such as visible light, and such converted light is read by the sensor. FIGS. 3A and 3B show an example of the X-ray image pickup device, wherein shown are circuit boards 2001; flexible circuit boards 2002; lead electrode portions 2010 provided on sensor substrates 2110-2113; and a sealant 2011.
The sealant 2011 is applied at the junction of the lead electrode portion 2010 and the flexible circuit board 2002, for the purpose of preventing corrosion of the lead electrodes 2010, and is generally composed of silicone resin. There are also shown sensor substrates 2003-2006 bearing thereon sensor elements 2007, a base member 2008 for supporting the sensor substrates 2003-2006, and an adhesive material 2009 for fixing the sensor substrates 2003-2006 to the base member 2008. The sensor substrates 2003-2006 are fixed to the base member 2008, with such alignment that the pitch of the pixels is matched two-dimensionally. There are further shown a fluorescent plate 2201 for converting the X-ray into the visible light, and an adhesive material for adhering the fluorescent plate 2201 to the sensor substrates 2003-2006. In FIG. 3A, the fluorescent plate 2201 is partially cut off at the lower right corner for the purpose of showing the structure thereunder. The fluorescent plate 2201 may be provided over the entire area corresponding to the area of the photoelectric converting elements 2007.
FIG. 4 shows an example of the process for preparing the X-ray image pickup device shown in FIGS. 3A and 3B. In a step (2301), sensor substrates 2110-2113 prepared by a thin-film semiconductor process are sliced into a desired size by a rotary diamond blade 2114. Then, in a step (2302), the circuit boards 2001 are connected, by suitable jointing means 2115 and through the flexible circuit boards 2002, to the lead electrode portions 2010 of the sensor substrate 2110-2113 prepared in the predetermined size.
In a next step (2303), the sealant is coated by a dispenser 2116 so as to cover the junction portion of the flexible circuit board 2002 connected onto the lead electrode portion 2010. Then, in a step (2304), the sensor substrates 2110-2113 after sealing are mutually so aligned that the pitch of the pixels matches two-dimensionally, and the substrates are placed on and fixed to the base member 2008 coated with the adhesive material in advance. Then the fluorescent plate 2201, coated with the adhesive material 2202 over the entire area for example by spraying, is placed on the sensor substrates 2110-2113. Then, in a step (2305), adhesion is achieved by a constant pressure applied by a roller 2203. In FIG. 4, the photoelectric converting elements (sensor elements) 2007 are omitted from the illustration.
In such solid-state image pickup device formed by arranging plural small-sized substrates, the image pickup device of pseudo single substrate is formed by matching the pitch of the pixels in two-dimensional directions. Consequently it is possible, when the small-sized substrate is completed, to inspect the electrical characteristics thereof, thereby judging each substrate satisfactory or defective, and to improve the yield at the next assembling step. On the other hand, it is necessary to pay attention, in the cutting process in the step (2101) or (2301) shown in FIG. 2 or FIG. 4, not only to the dimension precision and the positional error resulting from the burrs generated at the cutting, but also to the destruction of the elements resulting from the chipping at the cut portion and the corrosion.
FIG. 5 shows a connecting portion between the two neighboring substrates in a schematic plan view, wherein shown are sensor pixel portions 2401; thin film transistor portions 2402; gate lines 2403; and signal lines 2405. The interval of the gate lines or the signal lines corresponds to the pitch a of the pixels. In order to connect the neighboring substrates while matching the pitch of the pixels as shown in FIG. 5, the distance .beta. from the end 2406-1 or 2406-2 of each substrate to the gate line 2403-1 or 2403-2 has to satisfy a relation .beta.&lt;.alpha./2. In recent years, because of the requirement for a finer image and the advancement in the semiconductor technology, the pixel pitch of the solid-state image pickup device is in the order of several tens of microns to several hundreds of microns. With such miniaturization of the pixels, the distance .beta. from the end of the substrate to the gate line becomes shorter so that the size of chipping at the substrate end detrimentally affect the structure of the device.
FIG. 6 shows an example of a chipping at the connecting portion of the two neighboring substrates (upper part being a schematic plan view while lower part being a schematic magnified cross-sectional view), wherein shown are a chipped portion 2501; an insulating film (silicon nitride film) 2502; a protective film (polyimide film) 2503; and a transparent insulating substrate 2505. The insulating film 2502 and the protective film 2503 are normally so formed as to cover sensor pixel portions 2401, thin film transistor portions 2402, gate lines 2403 and signal lines 2405 to prevent intrusion of moisture and impurity ions such as of sodium, potassium and chlorine, thereby protecting the metal wirings from corrosion. However, if the substrate is chipped as illustrated to a position close to the gate line, the moisture and the impurity ions intrude at the interface of the insulating film 2502 and the transparent insulating substrate 2505, thereby inducing corrosion of the wirings and eventually leading to the breakdown thereof. This causes the lack of a line in the image, thus significantly deteriorating the image quality.
At the process of producing the X-ray image pickup device, in a state that the flexible circuit board is connected onto the lead electrode portion of the sensor substrate, a sealing material is painted by a dispenser to cover the junction portion. Then, the four sensor substrates after sealing are mutually so aligned that the pitch of the pixels matches in the two-dimensional directions, and are placed on and fixed to the base member on which the adhesive material is painted previously. And then, a fluorescent plate painted with the adhesive material over the entire area thereof for example by spraying is placed on the sensor substrates. And, pressing thereon at a constant pressure by a roller, the fluorescent plate is bonded to the base plate. In this operation, the excessive adhesive material is leaked out from the ends of the fluorescent plate, thereby further covering the lead electrode portions already covered with the sealant. As a result, the lead electrode portions are twice sealed.
The lead electrode portion or the surface thereof is covered with an Al film, or formed with Al or an Al alloy, and is therefore not protected against the moisture or the impurity ions such as sodium, potassium or chlorine. For this reason, the lead electrode portion is sealed, in a similar manner as explained above, by organic resin with low impurity ions, such as silicone resin or epoxy resin. Such resin is initially liquid and is solidified by a chemical reaction induced by the addition of heat, moisture, ultraviolet light or a hardening agent, and is provided with a considerable adhering power. Also between the sensor substrates and the fluorescent plate there is employed an adhesive material, which is required:
1. to be optically transparent; PA1 2. to have satisfactory adhesion to and to be hardly peelable from the substrates; PA1 3. to be of a low viscosity in order not to include bubbl es between the substrates; and PA1 4. to be hardenable at the room temperature, in consideration of peeling of the substrates by the difference in the thermal expansion coefficients.
These requirements apply also to the sealing resin for the lead electrode portions, so that a same material may be employed for both purposes.